SARP l/ll recognized clinical/pathologic differences of severe asthma compared to milder asthma and identified distinct severe asthma phentoypes at baseline. Additional pathobiologic abnormalities were observed to occur in and across clusters which linked similarities in symptoms, exacerbation predilection, treatment response. Yet, nothing is understood regarding the stability of implications of these clinical or pathologic phenotypes. Published SARP II data show that chymase positive mast cells (MCTC) predominate in the submucosa and epithelium in severe asthma, with evidence for an altered activation status. Preliminary data suggest that a luminal MCTC mRNA signature and activation pattern even better differentiates symptomatic and exacerbation prone severe from milder asthma. This MC signature is present across at least 2 of the 3 predominant severe asthma clusters. However, the mechanisms behind these changes, the interaction of these MCs with epithelial/inflammatory cells and their long term effects (and stability) are poorly understood. The goals of this application are to establish a longitudinal protocol capable of identifying asthma phenotypes and their long term implications in both adults and children with asthma and severe asthma, as well as evaluating their stability. This longitudinal protocol will intersect with mechanistic studies which identify a MCTC molecular phenotype, relate it to genetic characteristics as well as short/long term cellular, clinical, physiologic and radiologic outcomes and then analyze its stability over time. Finally, the proposal will mechanistically determine the impact of this mast cell signature on human airway epithelial cells. This innovative combination of in vitro/in vivo mechanistic and longitudinal molecular and clinical phenotyping is highly likely to uncover new molecular targets for severe asthma. RELEVANCE (See instructions): Severe asthma, impacts a minority of asthmatics, but accounts for a majority of the costs. While treatment of asthma has improved, severe asthma remains problematic and poorly treated. The proposed studies will identify new/novel molecular pathways, link them to baseline and longitudinal clinical, physiologic and radiologic outcomes and assess their stability over time leading to new molecular targets for therapy.